User blog:Samuli.seppanen/More thoughts on the cheiroballistra draw length
In previous blog posts (here and here) I've talked about cheiroballistra's draw length, or more correctly it's power stroke. My initial assumptions were that # When the cheiroballistra is cocked, the slider fills the entire female dovetail in the case. In other words, the slider is pulled as far as it goes. # The location of the claw from the end of the slider (5 dactyls) given in the manuscript is correct. With these assumptions the power stroke of the cheiroballistra is about 65 cm. As discussed earlier (here and here) reaching full 65 cm draw is fairly tricky, and any modifications to arm length, distance between field-frames or location of the little ladder will not make technical sense and/or violates the measurements in the manuscript. In fact, the only way to reach the full draw without causing excessive stacking is to make the arms point significantly (~30 degrees) outwards from the case at rest. Still, even with this modification the full draw is just barely achievable. So the question then becomes whether or not the draw was adjusted to be most suitable for each individual ballista? This was the case for the gastraphetes (stomach bow) which had the s.c. toothed boards and pawls to lock the slider at regular intervals. The cheiroballistra does not have anything like that, but the draw length could have been easily selected by placing the locking pin(s) for the handle appropriately, based on practical tests with any given machine. This would significantly reduce the need to make the depth of the curves and the distance between the field-frames exactly the same on every cheiroballistra. On one ballista the arms could be parallel to the case (at rest), and on another one they could point 30 degrees outwards, but the rest of the components would still work just fine. So far almost every detail in the cheiroballistra has made sense, with no need to amend any measurements which are not obviously wrong. In this context "obviously wrong" means measurements which contradict other parts in the text. There are only a few of these measurements, but still some scholars have "amended" the numbers to better suit their preconceived notions of what they think the cheiroballistra should have looked like, This is problematic, because it quite often causes a "ripple effect", i.e. once a measurement has been changed, the component will not work properly, or becomes out of balance with the other components. This will then require more fixes, which may require even more fixes, and the end result bears little resemblance to the original cheiroballistra as described by the manuscript. I've noticed that by simply sticking to the measurements in the manuscript this kind of problems can be avoided - except, of course, in the few isolated cases where the measurements are wrong. Even though I would have loved to see every single detail in the cheiroballistra to just fit perfectly, I will have to drop the "slider must be drawn fully" requirement to move forward. I will probably end up placing the arms at 20-25 degree angle and settle with 55-60 cm draw. This way I can start gauging the maximum energy storage of the cheiroballistra without having to deal with the excessive stack at the end. Category:Blog posts Category:Backup